The spindle I use is described in this post.
I do cut long 3D jobs in aluminum with it. But I don’t believe that spindle power is the limiting factor when milling aluminum with XCarve. The keys to success are (1) getting the flex and play out of the machine (2) choosing milling settings which avoid aluminum sticking to the bit (3) choosing an aluminum specific bit helps (4) oil on the cut can help get through a job if the bit starts to get too hot.
Also, many folks on the forum have reported settings which worked for them when milling aluminum with XCarve.